Dark current characterization of GaAs photoconductive switches is examined with an emphasis on low frequency current oscillations caused by travelling charge domains in the semiconductor. The voltage controlled negative differential resistance responsible for this phenomenon is due to field enhanced capture of deep level traps and is utilized for extraction of trap parameters using simple thermionic measurements. The GaAs switch investigated is of the avalanche variety and has been shown to produce current filament.ation in the on state. Since this latter effect is associated with a current controlled negative differential resistance region, we speculate on the nature of the transition between the two states.